Coating compositions and lithographic plates made therefrom

ABSTRACT

THE PRESENT INVENTION RELATES TO HYDROPHILIC COATING COMPOSITIONS FOR LITHOGRAPHIC PLATES COMPRISING, ZINC OXIDE, INERT FILLER, AMMONIA NEUTRALIZED POLYACRYLIC ACID AND STYRENE-BUTADIEN COPOLYMER. SUCH COATINGS OBVIATE THE NECESSITY FOR PRE-TREATMENT OF THE LITHOGRAPHIC PLATES BY BRUSHING AND/OR BY APPLICATION OF A BRUSH COAT OF A POLYVALENT METAL SALT. IN ADDITION, THEY ARE COMPLETELY WATERPROOF, POROUS AND LITHOGRAPHIC AND GIVE GOOD ADHESION OF IMAGES APPLIED WITH CARBON TYPEWRITTER RIBBONS.

Patented June 1, 1971 COATING COMPOSITIONS AND LITHOGRAPHIC PLATES MADE THEREFROM Arne E. Gornsi, Chillicothe, Ohio, assignor to The Mead Corporation, Dayton, Ohio No Drawing. Filed May 12, 1966, Ser. No. 549,491 Int. Cl. C08d 9/08, 13/16 US. Cl. 26041.5 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to hydrophilic coating compositions for lithographic plates comprising, zinc oxide, inert filler, ammonia neutralized polyacrylic acid and styrene-butadiene copolymer. Such coatings obviate the necessity for pre-treatment of the lithographic plates by brushing and/or by application of a brush coat of a polyvalent metal salt. In addition, they are completely waterproof, porous and lithographic and give good adhesion of images applied with carbon typewriter ribbons.

The present invention relates to hydrophilic coatings for lithographic plates. More particularly, it relates to hydrophilic basecoats for lithographic plates.

Lithographic surfaces have previously been prepared by applying to suitable carriers, coatings comprising casein and ester gum to function as a water receptive hydrophilic coating. The latter included such materials as casein, inert fillers and formaldehyde. and were applied in one or more layers depending upon the number of copies to be made from the lithographic plate. Following application and drying of the above coating, the surface was treated as by brushing and/or insolubilized by application of a wash coat consisting of a solution of a polyvalent metal salt, such as aluminum formate, aluminum chloride, zinc chloride or the like.

When it was desired to produce a long run lithographic plate or master, in accordance with prior practices, it was customary to pre-treat the paper base in order to render it substantially water resistant by applying a resinous varnish to the surface of the paper. The application of such water resistant coatings to the paper base, however, made the latter less receptive to application of hydrophilic coatings and, hence, required an intermediate coat in order to insure satisfactory anchoring with the fibrous base sheet. In producing lithographic plates by prior art processes, it has also usuallybeen necessary to apply from five to seven, or more, coats to produce a suitable lithographic plate for paper or the like.

If ordinary inexpensive paper such as kraft is utilized as a base, the paper will swell and change dimensions if water penetrates the surface coating and reaches the base. This will result in distortion of the image, in imperfect registration and in destruction of the paper master.

During printing, a lihographic plate is wet by application of water to the non-image areas. Accordingly, when an imaged plate is pressed against the rubber blanket of an offset copy machine, the ink which is on the image areas will come off onto the blanket while areas in contact with the non-image areas will remain clear since these areas of the plate are wet only with water and have no ink on them. The blanket is then pressed against a paper sheet to transfer the ink image to the paper,

In preparing a lithographic plate for printing, the plate is treated with an aqueous etch solution. If the aqueous solution works into the body of the plate, the surface of the plate may not be completely wetted by the solution fectly blank will show background discoloration or become darkened or toned. This incomplete wetting of the surface of the plate may also occur during printing.

If water penetrates the surface of the coating and renders the coating soluble under the image areas, the image will no longer hold onto the plate and tends to leave the printing surface. Yet, if the surface is so highly resistant to the imaging material that it fails to adhere properly, the same thing occurs. The material forming the image must adhere tenaciously to the plate and not come off wholly or in part or be destroyed by successive transfer of ink from the image area. Attempts have been made to overcome these difliculties by various methods, as by interposing a resinous water barrier between the paper base and the surface coating but this has not been found to satisfactorily prevent water from being drawn from the surface into the coating layers above the barrier surface.

Accordingly, an object of the present invention is to provide means for obviating the difiiculties previously inherent in paper base lithographic plates.

Another object of the invention is to produce and to provide a method for producing a new and improved lithographic surface.

Still another object is to produce a hydrophilic basecoat for lithographic plates.

A further object is to produce a coating which may be used as the primary topcoat for short run lithographic plates for direct imaging.

Other objects will become apparent from the description which follows.

Applicants new coating composition basically comprises zinc oxide, ammonia neutralized polyacrylic acid, an inert filler and a styrene-butadiene copolymer. In some instances, also, it is desirable to include zinc acetate in the. composition.

The inert filler used in applicants claimed composition can be clay, talc, diatomaceous earth, satin white, silica or other such pigments customarily used in lithographic plates, i.e., which should not react with the etching solution. The ratio of inert filler to zinc oxide is preferably of the order of 2.4 to 1.

The zinc oxide and inert pigments are bonded together on the lithographic plate by use of a combination of ammonia neutralized polyacrylic acid and styrene-butadiene copolymers. The polyacrylic acid should preferably have a molecular weight of at least 60,000 and is neutralized with ammonia to a pH of above 9.5. The preferred styrene-butadiene copolymer contains about 60% styrene and 40% butadiene and is added to the coating composition in emulsion form.

It has generally been found desirable for best results to use 8 to 10 parts of ammonia neutralized polyacrylic acid to l to 4 parts of styrene-butadiene copolymer, with optimum results when using 8 parts. of ammonia neutralized polyacrylic acid to 2 parts of copolymer. Ratios of 9-10 parts of ammonia neutralized polyacrylic acid to less than 1 part styrene-butadiene copolymer have been found to give poor pigment binding and ratios of 6 to 6, and 9 to 5 of ammonia neutralized polyacrylic acid to styrene-butadiene copolymer showed poor erasures.

The ratio of zinc oxide to adhesive should preferably be of the order of 1-4 to 1, and preferably 2.2-2.7 to 1 for best results.

Suitable supports for the lithographic plates may include paper, fiberboard, textile fabrics, plastic sheet stock or metal strips preferably thin enough to provide sufiicient flexibility for bending about a typewriter platen and at the same time having sufiicient strength to maintain dimensional stability for at least the number of copies to be produced therefrom.

When a fairly large number of copies is desired and the base stock comprises paper or other sheet stock of fibrous character having a high degree of water absorption, it is desirable to introduce high wet strength into the base stock by known pre-treatment with material such as resin solutions.

The specific examples which follow are given for the purpose of further illustrating applicants claimed invention however it will be evident that both the specific ingredients and the amounts thereof, as well as the method of mixing, can be varied. Any such variations which do not depart from the basic concept of the invention disclosed herein are intended to come within the scope of the appended claims.

EXAMPLE I The following compositions were prepared by high shear mixing of the zinc oxide and inert filler with about one-third of the ammonia neutralized polyacrylic acid and enough water to produce a thick paste. The remainder of the ammonia neutralized polyacrylic acid was then slowly added as a 27% solution of the acid in water, allowing suificient time for the polyacrylic acid to react completely with the zinc oxide pigment. The amounts of materials in this and subsequent examples are parts by weight of solids unless otherwise stated.

Zinc oxide-26.5 parts Hydraspere clay-63.5 parts Ammonia neutralized polyacrylic acid8.0 parts Styrene-butadiene copolymer (60-40% )-2.0 parts Sodium hexametaphosphate (dispersing agent)-0.l%

on pigments Total solids59.4%

Viscositypoises MacMichael at 75 F.

The above composition was blade coated at the rate of 4 pounds per 3300 square feet, onto a wet strength rawstock previously coated on the same side with 6 pounds per 3300 square feet of a starch-clay-zinc oxidemelamine resin composition. The resulting dried coating was completely waterproof and lithographic, as well as porous, resulting in good adhesion of images applied with a carbon typewriter ribbon. When tested on an A. B. Dick 350 press for lithographic characteristics, it gave more than 800 copies. In all respects, the master thus evaluated gave satisfactory results except that the etching solution soaked into the sheet rather rapidly causing toning for the first 25 copies when there was a time delay between etching and running on the press.

The above coating was porous and could readily be top coated with typical lithographic coatings and the adhesion described above was retained. When topcoated with a polyacrylate lithographic coating such as those shown and described in Examples 1-5 in copending application Ser. No. 413,057 filed Nov. 23, 1964, and now abandoned, and in a continuation application thereof, the copy life, ease of making corrections, appearance and press performance were excellent.

EXAMPLE II A coating of the following composition was prepared as above described:

Zinc oxide26.0 parts Hydraspere clay62. 0 parts Ammonia neutralized polyacrylic acid-8.0 parts Styrene-butadiene copolymer (60-40% )4.0 parts Sodium hexametaphosphate (dispersing agent) -0.25

on pigments Calcium stearate (lubricant)-0.25 on pigments The above composition was roll coated at the rate of 4% pounds per 3300 square feet onto non-tubbed 65# (25 x 38-500) wet strength rawstock.

After drying, a second coat of the same formulation was then applied by a blade at the rate of 3% pounds per 3300 square feet. The resulting dried coating was evaluated as above described and gave a good copy life of 300 copies, however, the coating gave ink specking at 200 copies and beyond, even though the copies were still legible at 300 copies.

EXAMPLE III The following composition was used in this experiment:

Zinc oxide26.5 parts Hydraspere clay-63.5 parts Ammonia neutralized polyacrylic acidparts Styrene-butadiene copolymer (60-40% )2.0 parts Sodium hexametaphosphate0. 1 5 on pigments Zinc acetate-5.2% on pigments Ammonia12.3 on pigments Total solids-62.6%

Viscosity-43.2 poises MacMichael at 75 F The composition was formulated as described above and approximately 4.25 pounds per 3300 square feet of the resulting product was applied by a blade coater to a wet strength rawstock previously coated with 6 pounds per 3300 square feet of a starch-clay-zinc oxide-melamine formaldehyde resin.

The dried sheet prepared as above described was then coated with a typical lithographic coating composition and supercalendered to improve the surface characteristics of the coated paper.

The sheet prepared as above described was then imaged with 29 different types of typewriter ribbons in order to more fully evaluate the lithographic properties of the sheet and then run on an A. B. Dick offset duplicator. Of the 29 ribbons tested, 25 gave a copy life of at least 500 copies. Under similar conditions, only 15 ribbons gave comparable results with lithographic plates prepared by prior methods, unless they had been previously washed with an aqueous solution of a polyvalent metal salt.

No specking of either the plate or copy occurred when the plate was run to 1500 copies.

The lithographic plate of the present invention, prepared as above described, was also found to function equally well when printing with magnetic inks. In making this test, the image characters and the magnetic inks were of the type specified for mechanized check handling by the Bank Management Commission of the American Bankers Association. Images were prepared using carbon ribbons backed with non fabric backings such as polyethylene terephthalate (Mylar) and polyethylene films, and the imaged plate was run on the duplicator using a specified magnetic ink. Excellent definition of the characters with no voids or ink smears was obtained in the copies produced.

The basecoat compositions described above have been found suitable for use as a basecoat for lithographic plates to be imaged electrophotographically, applicant's new basecoats being particularly useful for this purpose and eliminates the necessity for the separate barrier coat normally required between the electrophotographic coat and the basecoat customarily used in making such plates. Generally, the adhesives used in making electrophotographic plates for offset lithographic printing are water insoluble but are soluble in aromatic organic solvents, such as toluene. A barrier layer is therefore required to prevent the organic solvent from soaking into the basestock and producing a non-uniform electrophotographic coating and subsequent variations in the image on the surface. Applicants claim basecoat adequately prevents the entry of the organic solvent into the basestock and at the same time provides satisfactory anchorage of the electrophotographic coat to the base coat.

Applicants new basecoat can, therefore, be used with decided advantage for several different purposes such as used directly as a short-run lithographic plate, or as the basecoat for long-run lithographic plates to be imaged directly or by electrophotographic means.

What is claimed is: References Cited 1. Coating compositions for production of hydrophilic UNITED STATES PATENTS coatings comprising, zlnc oxide, an lnert filler, 8-10 parts by weight ammonia neutralized polyacrylic acid, and 1-4 3,349,704 10/1967 on "-5- 101-461 parts by weight styrene-butadiene copolymer. 5 3,230,873 1/1966 101-462 2. The composition of claim 1, wherein the inert filler 2,951,441 9/1960 Wolf et 101-462 is selected from the group consisting of clay, talc, diato- 2,760,431 8/1956 Beatty 101462 maceous earth, satin white, and silica.

3. The composition of claim 1, wherein the ratio of MORRIS LIEBMANPnmary Exammer inert filler to zinc oxide is of the order of 2.4 to 1. 10 L. T. JACOBS, Assistant Examiner 4. The composition of claim 1, wherein the ratio of adhesive to zinc oxide is l to 1-4. US. Cl. X.R.

5. A paper substrate having on the surface thereof a 155 dried coating composition of claim 1. 

